1. Field of the Invention
The present invention generally relates to implantable medical devices, and in particular to such devices capable of estimating analyte concentration in a subject body.
2. Description of the Prior Art
There is a great need in measuring different analytes in the animal and human body for diagnostic and medical purposes. Today, such analyte measurements are most often performed on blood samples taken from the subjects and analyzed in a laboratory environment. Although drawing a test sample of blood may be simple, it affects the subject's quality of life as he or she must often visit a healthcare facility for taking the sample. In addition, the equipment needed for analysis may be limited to hospital environments. As a consequence, some subjects must therefore make visits to the hospitals on a regular basis.
Several portable sensors implantable in the subject body or at least worn by the subject have been presented. Such sensors are most often based on the local measurement of an analyte in the immediate vicinity of the sensor or even in a measuring chamber of the sensor, through which blood flows. However, such implantable sensors often have significantly limited operational time. A major reason is the formation of connective tissue around the sensor or the measuring chamber. This connective tissue layer prevents or at least severely limits the transport of the analyte close to the sensor. As a consequence, these sensors therefore often become inoperable or highly unpredictable even after a very short operational period.
United States Patent Application Publication Nos. 2004/0127780 and 2004/0161881 and U.S. Pat. Nos. 6,841,389 and 6,517,482 disclose non-invasive determination of a substance, typically glucose, in blood of a subject. An electrical conducting probe is placed tight against a skin surface of the subject. Electric current is applied to the skin and underlying tissue using two electrodes of the probe. A resulting voltage is measured by the same or different probe electrodes. An impedance of the skin tissue can then be calculated based on the measurements and used for determining the blood glucose level in the subject.
Such a probe-based glucose measuring provides significant advantages to the subject by relaxing the need for taking blood samples, possibly at regular intervals, especially for a diabetic subject.